About 5 years ago I had the job to analyze options for Broadband Over Powerlines. The target applications was to provide cheap internet and data access to small towns.
I remember having read a lot of forums and there were a lot of concerns about RF interference (the options used OOFDM) and the inability of Power Lines to shield those RF emissions (as coaxial cables do and even coax cables do have power leakages). Power Lines (mainly made of copper and/or Aluminium) were not designed for such purposes and there were a lot of concerns because there were analysts talking that the noise floor will be increased affecting many existing communications (like public safety servicies; long range airplane communications, etc)
At that time it was a very risky move and the investors decided not to follow that direction. There were implementations but in-house or in-building (in france and spain) but not in the outside. The internet link was a DSL or ISDN and the internal distribution was done using the existing power lines (with about 2Mbps of data rate). Yes it was slow but very suitable for home-automation, building-automation and "intelligent homes".

I could see a few applications for this like remote control/sensing of street lights/traffic lights and weather conditions in a city. I do wonder about the interference issue and usefulness of broader communications/control using this method. I remember the fad a number of years ago with "smart houses" lots of smoke but no fire. I think there could be some interesting niche applications but I am not sure about general widespread use.

Bert22306 is entirely correct regarding RFI created by data over pwoer line systems. This has been a ten years plus conflict among licensed spectrum users and the illusion of easy savings and easy money by trying to send data of existing power lines. Most field trials of these systems have shown excess interference, insufficient real bandwidth for users and financial loss for the sstem operators. The narrow band systems are OK for in-home control, but the so-called wide-band systems are so far behind bandwidths users expect (50 Mb/s or more)that building such a system is like saddling a dying horse.

By reading the power line communication my mind went back to 1975. In my college days 3rd and 4th years of curriculam a miniproject to be done.One of this was PLC power line communication. That time all analog and i used AM, carrier frequency 150kHz.it worked within a same phase line in side the building. The modulation circuit uses a transistor and twin T filters (notch) rejection frequecy 50 Hz are used to couple the 150kHz signal to the main line.

Another article published in the EMC community is in http://www.nutwooduk.co.uk/pdf/Issue80.PDF and begins on page 25. This is a technical and regulatorial analysis for BPL in the UK. It highlights the main problems with BPL.

Broadband over Power Lines (BPL) has been documented to be a consistent source of radiated RF that interferes with other radio communication services. The FCC has also not enforced its own standards. See http://www.dslreports.com/shownews/New-Docs-Show-FCC-Glossed-Over-BPL-Flaws-102422
Devices sending broadband signals over elevated, unshielded power lines are not the answer to "simple" broadband communications.

Power line carrier (PLC) has been used for high-voltage power system communications since the 1950's. PLC has also been used in the home (120V/240V systems) for lighting control and security for many years.
Programmable logic controllers, PLC's as they are referred to now, were once called programmable controllers (PC's). The name was changed to avoid confusion with reference to desktop computers.
Also tne word is spelled acronym.

I see at least two problems with the idea of providing broadband service over PLC, that don't seem to have been mentioned.
The first is that the link layer service provided by powerlines would be shared by as many households as feed off any one transformer, no? It's not like you can easily isolate one household from the adjacent few, until you meet a transformer. So the bandwidth is necessarily shared, and presumably each household would have its own security codes to keep its internal network safe. (Not a big deal for low bandwidth functions like control of home lighting, of course.)
The second problem is that emissions from powerlines only attenuate as an inverse of the perpendicular distance from the powerline. Not the more common inverse square of distance, that you'd experience with point sources like WiFi or cellular telephony. So I would think preventing RF interference created by these PLC schemes will be more of a factor.